Nimrud took the lead with 75,000 people by 800 B.C.

Located in Iraq at a strategic point on the Tigris River, Nimrud was the capital of the Neo-Assyrian Empire.

In the ninth century B.C., King Ashurnasirpal II built a huge palace made of "cedar, cypress, juniper, boxwood, mulberry, pistachio wood, and tamarisk" filled with art and treasure. He also built temples, botanical gardens, and a zoo.

King Shalmaneser III, his son, would go on to build a palace that was twice as big.

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Nineveh took the lead with 100,000 people by 700 B.C.

Located in Iraq, Nineveh was settled as early as 6000 B.C. By 3000 B.C. it was a major religious center for the goddess Ishtar.

The city really took off around 700 B.C., when the Assyrian King Sennacherib built a "palace without rival." The city was spread over 7 square kilometers, with 15 great gates, 18 canals, and several sections of aqueduct.

Nineveh was besieged and sacked in 612 B.C. as the Neo-Assyrian empire crumbled.

Rome took the lead with 400,000 people by 100 B.C.

Founded around the eighth century B.C., Rome conquered the Italian peninsula and came to dominate the Mediterranean by the second century B.C. Julius Caesar's conquest of Gaul in the first century B.C. led to the establishment of the Roman empire.

The ruins of Ancient Rome are known around the world, from the Colosseum, where as many as 80,000 spectators watched gladiators fight, to the Forum, the site of parades, elections, and trade, to the Pantheon, a temple that has still the world's largest unreinforced concrete dome.

Rome grew to larger than 1 million people by 1 B.C. It remained the world's largest city until the fourth century, when the Western Roman empire crumbled.

Chang'an took the lead with 600,000 people by 600.

Located in north-central China, Chang'an was the capital of more than 10 dynasties. It became the largest city in the world under the Sui Dynasty as China emerged as the world's most powerful country.

Soon the city sprawled across more than 30 square miles, and by 700 it had a population of 1 million.

The poet Bai Juyi said the city had "hundreds of, thousands of houses — like a great chessboard … like a huge field planted with rows of cabbages." It had tree-lined boulevards five times as wide as New York's Fifth Avenue.

Chang'an declined sharply at the end of the Tang dynasty in 904. It lives on today, however, as the booming city of Xi'an.